1. Field of the Invention
This invention relates to adhesives for carpet backing which exhibit high delamination strength, both when wet and when dry. More specifically, the adhesives of this invention employ a copolymer dispersion of styrene, butadiene, and a mixture of ethylenically unsaturated carboxylic acid monomers. The adhesives are used to make recyclable carpets in which the adhesive can be completely separated from the carpet fibers, and the fibers re-used.
2. Description of Related Art
Carpet, particularly nylon carpet, is the floor covering of choice in many households and businesses in the United States and throughout the world. Unfortunately, carpet has a limited lifespan and must eventually be replaced, with the resultant used carpet waste generally being sent to landfill. Over recent years, the amount of carpet sent for disposal in the United States alone has increased to about 1.5 million tons per year. These vast quantities of carpet waste are burdensome to landfill capacity and have a negative impact on the environment. Furthermore, most carpet is made with nylon as the face fiber, a material that is relatively expensive. The quantity of used carpet discarded every year amounts to a loss of billions of dollars in potentially reusable nylon fiber.
To reduce the impact of used carpet on the environment, and to reclaim some of the financial loss due to discarding of fibers and other useful carpet material waste, carpet recycling would appear to be a logical solution. Recycling carpet, however, is difficult because its three major components are chemically and physically diverse. Most carpet consists of about 20-50 weight percent face fiber, the remainder being backing materials, commonly polypropylene, and adhesive which attaches the carpet fiber to the backing material. The adhesive typically comprises a carboxylated styrene-butadiene (XSB) latex copolymer, and an inorganic filler like calcium carbonate.
To recycle carpet, the face fibers are typically separated from the adhesive and backing to be reprocessed into new products or to be chemically recycled. Various methods for the mechanical removal of carpet fiber have been used. These methods disadvantageously result in low yield of the recycled carpet fiber. For example, U.S. Pat. No. 5,240,530 discloses a method of grinding carpet to a fiber length of less than about one-quarter inch and washing in a water bath to allow the various materials of the carpet to be separated by density.
In a further example, U.S. Pat. No. 5,230,473 describes a method for disintegrating, separating, and segregating whole carpet by loosening and debonding a latex/filler binder system by repeated application of highly pressurized fluids consisting of air, water, heated air, steam, and chemical solutions, and repeated stripping with rotating elements.
In another example, U.S. Pat. No. 5,722,603 describes a method of recovering face fiber from a carpet employing numerous steps of shredding and subjecting it to impact forces to reduce particle size, screen separating and washing the particles, followed by separation in water in a hydrocyclone. The washing operation optionally includes additives to improve the wetting and separation of the particles, such as sodium hydroxide and nonionic surfactant.
Another method to recycle carpet is to dissolve the carpet fiber itself from the remaining components. For example, U.S. Pat. No. 5,840,773 describes a method of extracting nylon from carpet waste by dissolving it in an alcohol-water agent. This method disadvantageously uses large quantities of organic solvent.
In another example, U.S. Pat. No. 5,889,142 describes a method of extracting nylon from carpet waste by dissolving it in a caprolactam-water mixture. This method also requires large quantities of organic solvent.
In a further example, U.S. Pat. No. 5,932,724 describes a method of depolymerizing multi-component waste material which is fed to a reactor as an extruded melt and contacted with superheated steam at high temperature and pressure to provide caprolactam which can be purified and polymerized.
Lastly, U.S. Pat. No. 5,916,410 describes the difficulty of recycling carpet fibers. A process involving organic softener is described.
To overcome problems of low purity and low yield associated with mechanical separation of fiber from carpet backing materials and the use of large quantities of organic chemicals, it is desirable to use a method involving an aqueous solution to remove backing materials from carpet face fibers. It is particularly desirable to develop an adhesive composition for carpet backing which binds carpet face fibers to the backing material, but which can be removed from the face fibers by applying shear in the presence of an aqueous solution, to permit efficient separation of the fibers from the adhesive and backing materials.
An adhesive composition must have high adhesive strength when dried to keep the backing and carpet attached, and must retain sufficient strength when wet to prevent premature failure of the carpet by separation of the fibers from the backing, for example during cleaning. However, while conventional adhesives for carpet backing typically have such dry and wet strengths, they are not easily removed from carpet fibers, but typically require extensive mechanical and chemical treatments to be removed. For example, U.S. Pat. No. 4,191,799 discloses a method for coating nylon carpet fiber with an aqueous curable adhesive that may be prepared from a copolymer of styrene, butadiene, and a carboxylic acid-containing monomer, combined with an olefin-grafted mineral oil extender. The adhesive can be prepared as an aqueous alkaline emulsion, but is not easily removed from carpet fibers.
In another example, U.S. Pat. No. 3,546,059 discloses a carpet fiber adhesive that may be prepared from styrene, butadiene, vinylidene chloride, and a functional monomer such as monoethylenically unsaturated carboxylic acid that improves the bonding of the fibers of the composite material. The latex binder is dried at 250xc2x0 to 400xc2x0 F.
Further, U.S. Pat. No. 4,009,310 discloses an adhesive for carpet fibers that may be prepared from a polymer of styrene, butadiene, methacrylic acid, and itaconic acid, combined with finely divided calcium carbonate and a thickener. The styrene is about 50 weight percent of the polymer, and the balance is a mixture of the remaining monomers.
In another example, U.S. Pat. No. 5,093,449 describes a latex bonding composition consisting essentially of styrene, butadiene, an ester, and methacrylic acid that improves dry bonding strength of carpet backing.
Thus, it would be advantageous to employ an adhesive for carpet backing that retains strength when wet, for example during cleaning, but can be completely removed by applying shear in the presence of an aqueous alkaline solution, without the use of organic compounds, to completely remove the backing material from the carpet face fiber to allow efficient recycling of carpet face fibers.
It is a technical advantage of this invention to provide such an adhesive. Another advantage of this invention is that it provides a method of using such an adhesive to prepare a carpet with recyclable face fibers, and a method to recycle the face fibers. The adhesive and carpet of this invention overcome the disadvantages of low yield in recovering carpet fibers and of using large amounts of organic solutions in recycling fibers. Further, they retain sufficient strength when wet to prevent premature failure of the carpet.
The carpet backing adhesive of this invention is prepared preferably using a carboxylated styrene-butadiene copolymer latex compounded with filler and other additives as known to those skilled in the art such as thickeners, foaming agents, and dispersants. The copolymer is prepared by emulsion polymerization and comprises monomers of styrene, butadiene, and an acidic monomer mixture which imparts to the adhesive high dry strength, high wet strength, and the ability to be totally removed from carpet face fibers to prepare a recyclable carpet. The adhesive can be removed from the face fibers by applying shear in the presence of an aqueous solution containing nonionic surfactant. The acidic monomer mixture comprises a mixture of ethylenically unsaturated carboxylic acid monomers. This acidic monomer mixture preferably comprises (1) a first ethylenically unsaturated monocarboxylic acid, and (2) a combination of a second ethylenically unsaturated monocarboxylic acid with an ethylenically unsaturated dicarboxylic acid. In a preferred embodiment, the first ethylenically unsaturated monocarboxylic acid is an alkylacrylic acid monomer, that is present in a quantity predominating the combination of the second ethylenically unsaturated monocarboxylic acid with the ethylenically unsaturated dicarboxylic acid.
In one embodiment, this invention is an adhesive for carpet backing produced as an aqueous dispersion that is compounded with filler.
In another embodiment, this invention provides a method of preparing a recyclable carpet using the adhesive compound to bind carpet backing to the carpet facing.
In another embodiment, this invention is a recyclable carpet which exhibits excellent delamination strength, both wet and dry.
In further embodiments, this invention provides a method of recycling carpet fibers from articles bonded with the adhesive described herein.
In another embodiment, this invention provides the recycled carpet face fibers.